Hermit crabs use certain molecules to track down new shells. With changes to ocean pH, their ability to do this is is compromised.
Photograph: Alamy

What if the way things smell started to change? What if food inexplicably lost its aroma and your house no longer had its familiar homely scent? It would certainly be off-putting, but you’d probably manage. However, many animals depend on their sense of smell much more than we do, so they would probably be affected much more acutely by a change in this key sense.

It seems that ocean acidification may be causing just such an alteration to the way that sea life smells the oceans. Or, more accurately, marine organisms’ ability to detect chemical signals is being altered by changes to ocean chemistry.

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We all know now that the CO2 emitted from burning fossil fuels is affecting the climate. Were it not for the fact that the oceans absorb about half of these CO2 emissions, the climate changes we are already experiencing would be far worse. But all that CO2 dissolving in the oceans comes with serious consequences. Once in water, CO2 forms carbonic acid, which in turn makes that water more acidic.

Humans’ effect on the chemistry of oceans is already measurable; the CO2 we have emitted since the beginning of the Industrial Revolution has caused the world’s oceans to drop from a pH of 8.2 to 8.1.

Now, that doesn’t sound like much, but the pH scale is logarithmic, so this actually means the oceans are 30% more acidic now than they were 200 years ago. If things carry on as they are, we can expect the oceans to be 150% more acidic by the end of the century, reaching pH 7.7.

Ocean acidification has an impact on the signalling and ‘smelling’ abilities of marine organisms. Illustration: University of Hull

We already know that in these increasingly acid oceans organisms struggle to make shells and that those shells may even start to dissolve (try dropping a seashell into dilute vinegar and see what happens). Meanwhile, organisms from clownfish to snails and crabs start behaving oddly in acidic waters. The fish wander too far from the safety of their protective anemones. The snails, dislodged by rough seas, fail to reattach to to rocks, and the crabs stop looking after their eggs. It has been suggested the acidic water affects these animals’ (and more besides) “sense of smell”. What this really means is that they can no can no longer detect the chemicals associated with their homes and so fail to “sniff” their way back from the dangerous open waters.

So far the hypothesis has been that the acidic water affects organisms’ chemical sensors, making them unresponsive to the molecules in the water. However, it turns out that this is by no means the whole story. My colleagues and I have found that the drop in pH actually directly affects the molecules being “smelt”. As the pH changes, the charge on the molecules alters and so does their shape.

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We tested how this might affect animal behaviours using shore crabs. When a female crab is carrying eggs she keeps them ventilated by wafting water around them. The female knows when to do this by “smelling” the state of her eggs. However in the low pH, when the molecules change shape, the crab doesn’t respond to the smell so well.

The consequences of this stretch far beyond shore crabs and their broods. The molecules we tested are also used by shrimp in a similar way. Meanwhile, barnacles and oyster larvae use them to find settlements, hermit crabs to track down new shells. And the problem may be even more far reaching. Marine organisms use many structurally similar chemicals for predator detection, food location and mate tracking, all of which are likely to be affected by ocean pH in comparable ways. As a consequence, the oceans of the near future may well smell very different from those of today, and marine eco-systems may not have time to adapt to its new scent.